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Earth and Planetary Science Letters 238 (2005) 248 – 268 www.elsevier.com/locate/epsl Constraining the extent of crust–mantle coupling in central Asia using GPS, geologic, and shear wave splitting data Lucy M. Flesch a,*, William E. Holt b, Paul G. Silver a, Melissa Stephenson a,c, Chun-Yong Wang d, Winston W. Chan e a Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, N.W., Washington DC 20015, United States b Department of Geosciences, State University of New York at Stony Brook, Stony Brook, NY 11794-2100, United States c Department of Earth and Atmospheric Sciences, Snee Hall, Cornell University, Ithaca, New York 14853, United States d Institute of Geophysics, China Earthquakes Administration, Beijing 100081, PR China e Multimax Corporation, 1441 McCormick Drive, Largo, MD, 20774, United States Received 16 August 2004; received in revised form 26 May 2005; accepted 6 June 2005 Available online 19 August 2005 Editor: R.D. van der Hilst Abstract We have obtained constraints on mechanical crust–mantle coupling for Tibet and Yunnan/Indo China, by comparing the observed surface deformation field inferred from GPS and Quaternary fault slip rate data, with the mantle deformation field inferred from several SKS shear wave splitting data sets. We first determined whether the anisotropy is dominantly asthenospheric or lithospheric by testing simple models of both types against the observed values of the fast polarization direction, /, which is assumed to be parallel to the horizontal projection of the direction of maximum shear. For asthenospheric flow, we solved for a best-fitting uniform sub-asthenospheric velocity model for Eurasia. The fit, however, was not satisfactory (RMS misfit D/ = 208). Solving for separate uniform flow fields in each region improves the fit (D/ = 158 for Tibet, D/ = 118 for Yunnan), although the resulting flow fields are inconsistent with several geophysical and geological constraints and thus considered unlikely. We then considered lithospheric models. For Tibet, vertically coherent deformation (i.e., maximum shear direction from surface deformation is parallel to /) yields an improved match (D/ = 118) for left-lateral shear. Both the goodness of fit and the dominance of left-lateral surface faulting in Tibet, argue for a lithospheric source of anisotropy. The misfit for Yunnan is large for either right- (D/ = 538) or left-lateral (D/ = 498) shear, which argues for complete crust–mantle decoupling in Yunnan. We show that the fast polarization directions throughout both the Tibet and Yunnan region can be fit by a single lithospheric dynamic model in which there is strong coupling between crust and mantle beneath Tibet, but a complete decoupling between crust and mantle beneath Yunnan crust. This dynamic model predicts left-lateral maximum shear directions within the mantle that align with fast polarization directions in both regions (D/ = 98). These maximum shear directions within * Corresponding author. E-mail addresses: [email protected] (L.M. Flesch), [email protected] (W.E. Holt), [email protected] (P.G. Silver), [email protected] (M. Stephenson), [email protected] (C.-Y. Wang), [email protected] (W.W. Chan). 0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2005.06.023 L.M. Flesch et al. / Earth and Planetary Science Letters 238 (2005) 248–268 249 the mantle align with the left-lateral maximum shear directions in the crustal deformation field in Tibet, but are not in Yunnan. Our results have the following implications. First, the coherence between crust and mantle deformation in Tibet implies strong crust–mantle mechanical coupling, since this is the only way that crustal buoyancy forces, required to account for the surface deformation field, can be transmitted into the mantle. This behavior is consistent with a uniform-strength lithosphere or strong crust, but not with a substantially weaker crust. For example, it is inconsistent with the popular bjelly-sandwichQ rheology, and thus precludes behavior such as large-scale lower crustal flow in Tibet. The crust–mantle coherence is also incompatible with mantle delamination. Second, crust–mantle decoupling within the Yunnan lithosphere argues that mantle deformation there is controlled only by boundary conditions; crustal buoyancy forces are not transmitted into the mantle beneath Yunnan. Moreover, the dynamic model for the mantle shows that the Yunnan crust is moving south with respect to the mantle at rates as high as ~30 mm/yr. Third, there is a fundamental rheological lithospheric transition between Tibet and Yunnan that may provide a key to understanding this significant orogen. D 2005 Elsevier B.V. All rights reserved. Keywords: lithospheric coupling; Tibet and Yunnan; shear-wave splitting 1. Introduction A fundamental rheological property of continents that controls their style of deformation is the depth dependence of lithospheric strength. Yet, this property is poorly constrained, a point that is best illustrated by the broad range of strength profiles, lithospheric behaviors, and boundary conditions adopted in modeling continental deformation [1–15]. Laboratory measurements of both crustal and mantle minerals within both the brittle and ductile regime have led to an almost universally accepted bjelly-sandwichQ strength profile for continental lithosphere (see Molnar and LyonCaen [16]). This model consists of an upper crust, controlled by Byerlee friction, a weak ductile middle crust, and possibly lower crust, predicted by the behavior of quartz rheology at high temperatures, and a strong upper mantle, predicted by laboratory experiments with olivine [17–20]. Despite the popularity of the bjelly-sandwichQ strength profile, the inferred behavior of continental lithosphere appears to be widely variable and can be thought of as having two end-members. In the first case the crust and upper mantle lithosphere are kinematically coupled and deform as a coherent unit [1–3]. Under these assumptions large regions have been modeled by solving the vertically averaged force balance equations using a vertically averaged rheology [21,22]. In these models the vertical coherence is attributed to the fact that upper mantle dry olivine rheology dominates the strength profile for continental lithosphere [23]. Hence the seismogenic crust responds passively to the motions occurring within the upper-most mantle [24]. A strength minimum in the middle and lower crust leads to the other end-member deformation mode, which involves a complete decoupling between the seismogenic crust and upper mantle and in many cases it involves lower crustal flow [10,25,26]. Recently, Maggi et al. [27] have proposed that the upper mantle beneath continents is aseismic and is weak relative the seismogenic crust. This model is at odds with the bjelly-sandwichQ model and suggests that the strength of the continental lithosphere resides within the seismogenic portion of the crust [28]. On the other hand, Chen and Yang [29] have observed mantle earthquakes beneath Tibet, providing support for a seismogenic, and potentially strong, upper mantle lithosphere there. The rheologies discussed above control how the lithosphere responds to the applied driving forces. Whereas the forces induced by a collisional boundary force are expected to be applied throughout the depth extent of the lithosphere, and therefore produce vertically coherent deformation for either rheology, the response to body forces associated with density variations is a function of whether or not there is mechanical coupling between the crust and upper mantle. For example, if topography is primarily compensated via the Airy mechanism, then body forces associated with lateral variations in topography are contained within the crustal layer, and will either be transmitted through the entire lithosphere (mechanically coupled) or only through the crust (decoupled). If there is coupling between crust and upper mantle and the 250 L.M. Flesch et al. / Earth and Planetary Science Letters 238 (2005) 248–268 crust is at least as strong as the upper mantle, then the mantle deformation field will respond passively to the dynamics that control crustal motions. On the other hand, if the upper mantle is strong relative to the seismogenic crust, and there is lithospheric coupling, then body and boundary forces applied to the mantle portion of the lithosphere will have a dominant control and signature on the surface deformation field. The key to discriminating between the different rheological profiles is the quantitative measurement of both the surface kinematics and mantle kinematics, and quantitative modeling of crust and mantle dynamics in response large-scale body forces and collision boundary conditions. The kinematics of crust and mantle can be inferred from analysis of two data sets: the observed surface deformation field inferred from geodesy, seismicity, and Quaternary fault slip rates (e.g., [30–32]), and observed mantle deformation inferred from mantle anisotropy (e.g., [33–38]). In this study we investigate the relationship between mantle deformation inferred from shear wave splitting measurements in Tibet, and within the Yunnan Province/Indo China region (referred to herein as Yunnan), which lies to the southeast of Tibet, and the continuous surface deformation field determined from GPS data and Quaternary fault slip rate data. We utilize a three-step process. First, we quantify the surface deformation field, using all available data to date. Second, we dynamically model the crustal and mantle deformation fields considering the two primary sources of stress: collisional boundary forces and the topographically induced body forces. Third, we compare these modeled deformation fields to the mantle field inferred from SKS shear wavesplitting measurements, and use any similarities or differences to ascertain the extent to which the crust and mantle are affected by the same forces. As we will show, we observe a distinct change in the rheology between Tibet and Yunnan. In the case of Tibet, the coherence of the crustal and mantle deformation fields shows that both sources of stress, topographically induced stresses and collisional boundary conditions, are transmitted throughout the depth range of the lithosphere. In particular the transmission of topographically induced body forces argues for a mechanically coupled lithosphere. In contrast, in Yunnan, while both the crust and mantle appear to respond to the collisional boundary conditions, the stresses associated with body forces are limited to the crust, thus arguing for an effective decoupling between the crust and mantle in Yunnan. The strong contrast in vertical rheological profiles between these two regions argues for a fundamental rheological boundary at the edge of the Tibetan Plateau. 2. Quantification and modeling of the surface deformation field in Asia Deformation in central Asia is unique in that it encompasses the largest zone of diffuse continental deformation on the surface of the Earth [16,39,40]. It exhibits the Earth’s highest topography, with the Tibetan Plateau possessing an average elevation of 5 km, and an average crustal thickness of 65–70 km. In addition, this region exhibits a variety of deformational styles. There is a fanning of compression around the Tibetan Plateau, with orientations of principal axes of compression parallel to the topographic gradient [3], and roughly N–S compression in the Tien Shan. There is also E–W extension occurring in southern Tibet [41–44] with principal axes of extension that rotate around to N–S extension east of the eastern syntaxis and back again to E–W extension in Yunnan, China [45]. The extension in Yunnan is accommodated by a series of NE–SW striking leftlateral strike slip faults, NW–SE striking right-lateral faults, as well as N–S striking grabens [45]. These faults are bounded by the Sagaing fault to the west that is a N–S trending right-lateral strike slip fault [46]. In addition, there has been past subduction of the Burma plate beneath the Yunnan/Indo China region [47]. With the advent of space-based geodetic measurements, the understanding of how strain is accommodated across this broad zone of deformation has increased dramatically [30–32]. This new constraint on the surface deformation field in Asia, together with information on mantle anisotropy, provides an excellent opportunity to investigate the vertical dependence of lithospheric deformation and its relation to rheology. The surface deformation field likely represents deformation down to a depth of at least 15 km and is thus a good proxy for deformation of the upper crust. To determine a continuous surface strain rate and velocity field, we follow the method of Haines and L.M. Flesch et al. / Earth and Planetary Science Letters 238 (2005) 248–268 Holt [48], which uses estimates of strain rates from Quaternary fault slip rate data (both rate and style) and GPS measurements, to invert for a continuous velocity and strain rate field [31]. The kinematic model is treated using a rotation vector function x(x̂), expressed as bi-cubic interpolation [49] on a curvilinear grid (Fig. 1) [50] that describes the horizontal components of a continuous velocity field u(x̂) uðx̂ x Þ ¼ rxðx̂ x Þ x̂ x ð1Þ where r is the radius of the Earth, and x̂ is the position of the unit radial vector on the Earth’s surface. We assign the spatial derivatives of the rotation 251 vector function x(x̂) to zero on what we define as the Indian and Eurasian plates to provide the constraint of rigidity for those plates. Even though we are modeling the horizontal components of the strain rate and velocity field, this modeling allows for thickening and thinning of the lithosphere (i.e., ė xx + ė yy = ė zz) (see Holt et al. [31] for details). We use Quaternary fault slip rates [30–31], and geodetic data [32,51–63], to determine a continuous self-consistent strain rate and velocity field model (Fig. 2). The Kostrov [64] moment rate tensor summation is used to determine strain rate estimates from Quaternary fault slip rate estimates. The calculated kinematic strain rate and velocity fields are similar to Fig. 1. The SKS shear wave splitting measurements used in this study; data in Tibet are from McNamara et al. [37]; Hirn et al. [34]; Guilbert et al. [33]; Sandoval et al. [38] (blue bars) and Huang et al. [36] (green bars). Data in Yunnan (yellow bars) are from this study, yellow stars indicate the location of 9 seismometers installed, and blue stars the two permanent stations. Also shown is the curvilinear grid used in the modeling (red region), which is defined by the geologic structures. Light blue lines represent Quaternary faults [30] used in the kinematic modeling. Black outlines delineate lithospheric blocks used in the mantle deformation forward models (TB=Tarim Block, G=Gobi Block, O=Ordos Block, S.C.=South China Block, S=Sunda Block). The solid black line over the Indian plate denotes the boundary of what we model as the rigid Indian plate and the solid black line over the western and northern sections of the grid denotes the rigid Eurasian reference frame. 252 L.M. Flesch et al. / Earth and Planetary Science Letters 238 (2005) 248–268 84 86 88 90 92 94 96 98 100 102 104 40 40 38 38 36 36 34 34 32 32 30 30 28 28 26 26 24 24 22 22 20 20 18 18 16 16 30 mm/yr 84 86 88 90 92 94 96 98 100 102 104 Fig. 2. The continuous velocity field (black vectors) determined from the joint inversion of GPS data (white vectors) [32,51–59,61–63] and Quaternary fault slip rate data [30,31] plotted relative to a Eurasian reference frame fixed. The ten spatially averaged SKS splitting measurements are plotted as blue bars. There is no GPS information surrounding the most northern point on the Huang et al. [36] line. that determined by Holt et al. [31], and yield a reduced chi squared of ~1.2 indicating a good fit to the GPS data. 3. Quantification and modeling of the dynamics of lithosphere in Asia Flesch et al. [12] have shown that this surface deformation field in central Asia can be successfully modeled by a relatively simple dynamic model that consists of roughly equal contributions from two driving forces: those associated with a collisional boundary condition resulting from relative plate motions, and those associated with gravitational potential energy (GPE) produced by the high topography of the Tibetan plateau. An important result from this dynamic modeling is that stress field boundary conditions and GPE variations produce distinctly different stress fields. The stress boundary L.M. Flesch et al. / Earth and Planetary Science Letters 238 (2005) 248–268 condition is consistent with a spatially uniform mostly N–S directed compressive stress associated with India’s convergence with Asia. However, deviatoric stresses associated with GPE variations are responsible for the spatial variation in principal axes of deviatoric stress and strain around the Tibe- 84 86 88 90 92 tan Plateau — compression parallel to topographic gradients in lowlands, roughly E–W extension throughout much of Tibetan plateau, N–S extension in southeastern Tibetan plateau, E–W extension in Yunnan province, and a strike slip style of deformation at intermediate elevations throughout the region. 94 96 98 100 102 104 A 40 253 40 38 38 36 36 34 34 32 32 30 30 28 28 26 26 24 24 22 22 20 20 18 18 Hotspot Reference Frame 16 16 84 86 88 90 92 94 96 98 100 102 104 Fig. 3. A) The SKS splitting measurements (black bars), the continuous velocity field (Fig. 2) plotted in a hotspot reference frame (white bars), and the velocity field plotted at the SKS splitting measurements (white bars with black outline). There is no correlation between the splitting measurements and the velocity field in the hotspot reference frame. D/ a = 588. B) The same as (A) only with the Eurasian velocity plotted in the no-net-rotation reference frame. D/ a = 208. C) The best-fit rotation (65.8, 104.1, 0.388/my) for the sub-asthenosphere (white vectors) and the surface velocity (black vectors) determined from GPS measurements and Quaternary fault slip rates, plotted relative to a fixed Eurasian reference frame. The difference in these two vectors (grey bars) should be parallel to the observed SKS splitting measurements (black bars) if a simple asthenospheric flow hypothesis is the origin of the anisotropy. D/ a = 188. 254 L.M. Flesch et al. / Earth and Planetary Science Letters 238 (2005) 248–268 84 86 88 90 92 94 96 98 100 102 104 B 40 40 38 38 36 36 34 34 32 32 30 30 28 28 26 26 24 24 22 22 20 20 18 18 NNR Reference Frame 16 16 84 86 88 90 92 94 96 98 100 102 104 Fig. 3 (continued). Therefore GPE variations are responsible for the curvature of active left-lateral faults around the Tibetan Plateau (e.g., Kun Lun Fault, Xianshuihe Fault) and for the dramatic curvature of surface velocity observed around the eastern Himalayan syntaxis into Yunnan (Fig. 2). If, to first order, the topography beneath Tibet is supported by crustal thickness variations (e.g., [65]) then the bulk of the horizontal density variations (GPE variations) responsible for the spatial variation of the surface deformation field described above are embedded within the crust, not the mantle [16]. 4. Shear wave splitting measurements in Asia: analysis and interpretation Since anisotropy is related to the finite strain tensor [66], with the a-axes of polycrystalline olivine aggregates tending to align parallel to the direction of maximum shear for strain greater than 75% and the maximum extension direction of finite strain for strain less than 75% [67], mantle anisotropy provides a means of constraining the orientation of the mantle deformation field. An extensively utilized manifestation of mantle anisotropy is shear wave splitting as L.M. Flesch et al. / Earth and Planetary Science Letters 238 (2005) 248–268 84 86 88 90 92 94 96 98 100 102 104 C 40 255 40 38 38 36 36 34 34 32 32 30 30 28 28 26 26 24 24 22 22 20 20 18 18 Athenosphere surface 16 16 30 mm/yr 84 86 88 90 92 94 96 98 100 102 104 Fig. 3 (continued). measured from near-vertically propagating shear waves such as the phase SKS. In this geometry, the fast polarization direction, /, is parallel to the direction of maximum shear ([68] and references therein). The question we seek to ask from the splitting observations is the extent to which the surface deformation, and by implication the forces that produce this deformation, extend into the mantle. Silver [68] argued for vertically coherent deformation beneath Tibet based on the observation that the SKS shear wave splitting measurements correlated with the orientation of surface faults (i.e., vertical shear planes). Davis et al. [69] used thin viscous sheet methodology to model deformation within central Asia, and found that for Tibet, the direction of maximum extension was within 308 of the fast polarization direction predicted from the SKS splitting measurements. They used this to argue for vertically coherent lithospheric deformation. Assuming that dynamic recrystalization plays a role in the development of the mantle anisotropy (in which case the aaxis of olivine would follow the direction of maximum shear [67]), Holt [70] found that the direction of maximum left-lateral shear in the horizontal velocity 256 L.M. Flesch et al. / Earth and Planetary Science Letters 238 (2005) 248–268 field of Tibet, determined from Quaternary fault slip rate data and somewhat limited GPS data, was remarkably close to the fast polarization directions from the shear wave splitting results, producing an RMS variation of 108–158 (1r). This correlation suggested that planes of finite left-lateral shear in the mantle beneath Tibet are oriented parallel to the active planes of finite shear (faults) within the crust. Holt 84 86 88 90 92 [70] concluded that this correlated deformation between crust and upper mantle was the result of these two levels of the lithosphere being subject to the same collisional boundary conditions. We make use of six shear wave splitting data sets for this study. For Tibet we use previously published SKS shear wave splitting data from McNamara et al. [37]; Hirn et al. [34]; Guilbert et al. [33]; Sandoval et 94 96 98 100 102 104 40 40 38 38 36 36 34 34 32 32 30 30 28 28 26 26 24 24 22 22 20 20 18 18 Surface Deformation Field 16 16 84 86 88 90 92 94 96 98 100 102 104 Fig. 4. The continuous surface deformation field associated with the velocity field shown in Fig. 2, plotted as the left-lateral maximum shear direction of deformation (white bars). Also plotted are the right-lateral (grey bars) and left-lateral (white bars black outline) maximum shear directions at the SKS splitting measurements (black bars). There is a correlation between the SKS splitting measurements and the left-lateral maximum shear directions in Tibet (D/ s = 118) and no correlation with the left-lateral maximum shear directions in Yunnan (D/ s = 498). There is no correlation between the right-lateral maximum shear directions in either region and the SKS splitting measurements in either region (D/ s = 778, Tibet; D/ s = 538, Yunnan). L.M. Flesch et al. / Earth and Planetary Science Letters 238 (2005) 248–268 al. [38]; and Huang et al. [36] (Fig. 1). We spatially average the 57 splitting measurements (Fig. 1) for Tibet into 9 regional measurements (Figs. 3–8) so as to give equal weight in our models to each area in 84 86 88 90 92 257 Tibet. We only choose regions where there is both well-constrained surface deformation and splitting data. There are several GPS measurements (Fig. 2) along the McNamara et al. [37]; Hirn et al. [34]; 94 96 98 100 102 104 40 40 38 38 36 36 34 34 32 32 30 30 28 28 26 26 24 24 22 22 20 20 18 18 Modeled Mantle Deformation Field 16 16 84 86 88 90 92 94 96 98 100 102 104 Fig. 5. The left-lateral maximum shear directions (white bars) of the continuous modeled mantle deformation field resulting from rigid body rotations of the Indian plate, the Tarim Basin, Ordos Block, the Gobi platform, the south China block, and the Sunda block relative to a stable Eurasia and assuming uniform viscosity for upper mantle lithosphere. We assume that the leading edge of the Indian plate is located in southern Tibet. We assume that these blocks are rigid throughout the lithosphere. Therefore with knowledge of the angular velocities of the blocks at the surface from GPS measurements, the angular velocities of the mantle portion of these blocks are also known. Also plotted are the left-lateral (white bars black outline) and right-lateral (grey bars) maximum shear directions of the modeled mantle deformation field at the SKS splitting sites, along with the SKS splitting measurements (black bars). We observe correlation between the modeled deformation field and the left-lateral maximum shear direction in Yunnan (D/ m = 88), but the fit to the left-lateral maximum shear directions in Tibet (D/ m = 218) is degraded from the fit to the surface deformation field. Again, there is no correlation for the right-lateral shear plane for either case (D/ m = 488, Tibet; D/ m = 828, Yunnan). 258 L.M. Flesch et al. / Earth and Planetary Science Letters 238 (2005) 248–268 84 86 88 90 92 94 96 98 100 102 104 40 40 38 38 36 36 34 34 32 32 30 30 28 28 26 26 24 24 22 22 20 20 18 18 Hybrid Mantle Deformation Field 16 16 84 86 88 90 92 94 96 98 100 102 104 Fig. 6. The left-lateral maximum shear directions (white bars) of the hybrid mantle deformation field that were determined using the method described in Fig. 5 with an additional surface boundary condition placed over the shear wave splitting measurements in Tibet to simulate a coupling between the crust and mantle layers. The left-lateral (white bars black outline) and right-lateral (grey bars) maximum shear directions are plotted at the splitting sites, along with the shear wave splitting measurements (black bars). There is an excellent fit between the modeled mantle deformation field and the observed mantle deformation from shear wave splitting in both Tibet (D/ m = 108) and Yunnan (D/ m = 98). Guilbert et al. [33]; Sandoval et al. [38] lines (blue bars, Fig. 1) and therefore the surface deformation there is well known. Along the Huang et al. [36] line (green bars, Fig. 1) there is no GPS coverage (Fig. 2), only GPS observations on either side of the southern part of the splitting observations. This splitting data shows a rapid spatial variation in / from the NE–SW in the south to NW–SE in the north, a rapid variation not observed in our surface deformation model, possibly due to lack of GPS data there to constrain it. Thus, we give results without the northern most splitting observations to only compare the mantle anisotropy with known surface deformation. For the Yunnan China region we utilize new SKS shear wave splitting measurements, which represent data from an eight-month deployment from 08/2000 to 04/2001 of 9 STS-2 broadband seismographs as shown in Fig. 1. The technique of Silver and Chan L.M. Flesch et al. / Earth and Planetary Science Letters 238 (2005) 248–268 84 86 88 90 92 94 96 98 100 102 259 104 40 40 38 38 36 36 34 34 32 32 30 30 28 28 26 26 24 24 22 22 20 20 18 18 mantle surface 30 mm/yr 16 84 86 88 90 16 92 94 96 98 100 102 104 Fig. 7. The modeled surface velocity field (black vectors) determined from the joint inversion of GPS and Quaternary fault slip rate data and the modeled mantle velocity field (white vectors) that yield the left-lateral maximum shear directions shown in Fig. 6. The mantle velocity field was derived assuming a uniform viscosity upper mantle lithosphere with applied side boundary conditions of rigid geologic blocks along with an approximated top boundary condition associated with crustal motions (Fig. 2), applied only to Tibet (see text for details). There is significant differential horizontal velocity between crust and upper mantle, starting around the eastern Himalayan syntaxis and throughout Yunnan. [71], as well as the stacking procedure of Wolfe and Silver [72] was used to analyze the data. 7 of the 9 stations provided usable splitting data. The stacked results are shown in Table 1, and the specific events and phases used for each station are given in Tables T1 and T2, and the quality of the station stacks is illustrated in Figs. S1 and S2, including the results of two permanent stations CHTO and KMI. As seen in Fig. 1, the orientations are surprisingly uniform. All stations give a consistent E–W oriented fast polarization direction with delay times that range from 0.5 to 1.7 s. 4.1. Lithosphere or asthenosphere? We first seek to test simple models constrain the dominant process that is producing the anisotropy. As in all shear wave splitting studies using teleseismic 260 L.M. Flesch et al. / Earth and Planetary Science Letters 238 (2005) 248–268 Fig. 8. A) The east–west (horizontal) shear strain rates (ė xz) produced as a result of the differential horizontal velocities between crust and mantle, shown in Fig. 7. B) The north–south (horizontal) shear strain rate (ė yz) produced as a result of the differential horizontal velocities between crust and mantle, shown in Fig. 7. Note the large shear strains in Yunnan China where the crust is moving southeast relative to Eurasia but the underlying mantle is moving NE. shear waves, the splitting observed at the surface can be due to either asthenospheric flow [73–75] or deformation of the lithosphere [68,71]. We can distinguish between these two possibilities by comparing the success of predictions of / under the assumption of a lithospheric or asthenospheric source. We first test the hypothesis that there is negligible lithospheric anisotropy and that the anisotropy is within the asthenosphere due to simple asthenospheric flow [68]. In this case the orientation of / will be parallel to the differential velocity vectors defined by the velocity at the top and bottom of the asthenosphere. In this test we furthermore assume that the surface velocity field is equal to the velocity at the base of the lithosphere (vertical coherence) and then specify a lithospheric velocity relative to a deep mantle frame. We consider two commonly held assumptions about the motions of the surface plate velocity field relative to deeper mantle: the hotspot frame and the no-net-rotation (NNR) reference frame. To evaluate the asthenospheric flow model for these two reference frames, we rotate the velocity field shown in Fig. 2 into the hotspot reference frame with the pole of rotation given by Gripp and Gordon [76] ( 44.8, 58.1, 0.098/my) and a NNR reference frame with the rotation pole from Argus and Gordon [77] (50.6, 112.4, 0.248/my). The resulting differential velocity vectors, which should be parallel to the orientation of /, are plotted in Fig. 3A and B L.M. Flesch et al. / Earth and Planetary Science Letters 238 (2005) 248–268 Table 1 The Yunnan, China splitting measurements Station / (8) r/ (8) yt (s) r yt (s) Latitude (8) Longitude (8) CHTO DY JC KMI LC LK ML NJ NL 105 102 000 94 102 93 87 87 000 2 9.5 000 8.1 3 2 3.5 4.5 000 1.65 1.23 000 0.56 1.33 1.25 0.88 0.95 000 0.05 0.3 000 0.10 0.10 0.08 0.08 0.10 000 18.79 25.73 26.54 25.12 23.88 25.83 21.48 25.05 27.29 98.98 101.32 99.90 102.74 100.07 98.85 101.56 100.52 100.84 The fast polarization is represented by /, yt is the delay time, and r / and r yt are the 1r uncertainties in / and yt respectively. Null stations are indicated by 000. for both reference frames. The RMS misfit for the simple asthenospheric flow models, D/ a, is very large for the hotspot reference frame (D/ a = 588), which was also noted by Davis et al. [69]. For the NNR frame, while D/ a is lower (D/ a = 208), there remain systematic misfits that are much larger than the errors in the splitting data, which are about 58–108 (Table 2). There remains the possibility that the sub-asthenospheric velocity field has some other velocity. Indeed, there are two studies where simple asthenospheric flow dominates the anisotropy [74,75], but the inferred sub-asthenospheric flow field is non-zero in both of the frames adopted. Moreover, the reference frame for Eurasia relative to the deep mantle frame is uncertain. Therefore, following the method of Silver and Holt [74] we invert for the best-fitting reference frame that will align the motion of the lithosphere with the splitting observations in Asia (Fig. 3C; Table 2). The best-fit rotation pole applied to the Eurasian frame is given by (43.5, 88.0, 0.418/my). The resulting shear directions between lithosphere and deeper mantle frame do not provide an especially good fit to the splitting observations over all of Asia (D/ a = 188). It is possible to have two distinct flow models (one for Tibet and one for Yunnan). Because of the uniformity of the splitting observations in Yunnan, such a model is consistent with the Yunnan observations (D/ a = 118) with a pole relative to Eurasia of (39.2, 98.8, 1.08/my) that again produces a uniform westward-directed sub-asthenospheric mantle velocity of ~30 mm/yr. In Tibet, solving for a single pole of 261 rotation also produces an acceptable fit to the splitting data (D/ a = 158) for a pole of rotation relative to Eurasia of ( 29.7, 92.8, 0.908/my). The resultant velocity field produces NNW velocities of ~30 mm/yr in eastern Tibet (in a Eurasia-fixed reference frame) that rotate around to WSW velocities, ranging from 1– 15 mm/yr in central and western Tibet. Although the fit to the shear wave splitting data is acceptable in Tibet and Yunnan, we reject the hypothesis that two independent asthenospheric flow fields are responsible for the observed anisotropy in Asia for the following reasons. First, these two predicted flow fields are divergent at the eastern Himalayan syntaxis, corresponding to ~6 cm/yr of relative divergence within the sub-asthenospheric mantle flow field. If this were the case, there should be manifestations of such an upwelling at the surface. Second, the estimated subasthenospheric flow for either region is not consistent with the motion of the subducting Indian plate. In a Eurasia-fixed frame, the velocity of the Indian plate beneath Tibet is northeast, whereas the estimated flow direction is northwest. In Yunnan, the best-fitting velocity is westward, while the subduction direction is northeast. In addition, these velocities are not consistent with any large-scale deep mantle flow model. We note that in two regions where simple asthenospheric flow appears to dominate, such flow models predict non-stationary sub-asthenosphere flow which successfully account for the observed anisotropy (e.g., [74,75]). The limited success of simple asthenospheric flow models motivates our consideration of lithospheric models for the anisotropy. The simplest to such model is vertically coherent deformation [68], where Table 2 The RMS misfit for the 5 different models to the shear wave splitting measurements RMS misfit to: Tibet Yunnan Both regions Hotspot model NNR model Active mantle (1 rotation) Active mantle (2 rotations) Surface right-lateral Surface left-lateral Mantle right-lateral Mantle left-lateral Hybrid right-lateral Hybrid left-lateral 56.38 18.48 24.18 15.08 76.68 11.28 48.38 20.58 66.08 9.58 60.98 21.48 13.98 11.38 53.08 49.08 81.78 8.18 79.38 9.08 58.38 19.78 18.18 – 67.38 33.48 65.18 16.38 72.18 9.38 262 L.M. Flesch et al. / Earth and Planetary Science Letters 238 (2005) 248–268 the crust and mantle portions of the lithosphere deform coherently with no horizontal shear, as would be the case for a mechanically coupled lithosphere. In this case, the direction of maximum shear for a highly strained region like Tibet for the surface field should be parallel to the orientation of / from the splitting measurements [67]. To test this hypothesis we compare the surface deformation field associated with the velocity field shown in Fig. 2 with the shear wave splitting observations. This involves comparing the maximum shear directions from the modeled instantaneous strain rate field, determined using GPS and Quaternary fault slip data, / s, with SKS fast polarization directions, inferred to represent finite maximum shear directions in lithospheric mantle. The maximum shear direction is independent of reference frame so that our comparison applies for all possible reference frames. We plot the right- and left-lateral maximum shear directions [70] embedded in the velocity gradient tensor field (Fig. 2) and compare these shear directions with the fast polarization directions (Fig. 4). For the Tibet region, while there is a large misfit observed between the surface and mantle deformation for the right-lateral case, D/ s, (D/ s = 778), we observe a close correspondence between the left-lateral shear directions and values of / (D/ s = 118) (Fig. 4; Table 2). Note that the left-lateral shear planes in the instantaneous deformation field are parallel to the left-lateral faults in Tibet, which are features of finite strain. The correlation of / with the left-lateral shear planes in the deformation field makes several important points. First, the anisotropy-inferred mantle deformation within Tibet is most likely lithospheric. Second, the lithospheric deformation is vertically coherent with the surface, since the surface deformation field (leftlateral direction of maximum shear) match so closely with the fast polarization directions, and hence the inferred mantle finite shear directions [67,70]. Third, the left-lateral maximum shear direction is a better predictor of / than the maximum extension direction of finite strain. The RMS misfit found by Davis et al. [69] using the modeled maximum extension direction (D/ = 198) is almost twice as large as for this study. Much of the systematic misfit that Davis et al. [69] observed can in fact be dramatically reduced by adopting the direction of maximum shear rather than maximum finite-strain extension direction. Fourth and most importantly, since the dynamic modeling of the surface deformation field requires ~50% contribution from body forces contained within the crustal layer [12], it follows that the stresses associated with crustal GPE variations must be transmitted into the mantle to produce the observed coherent deformation field, which argues for strong crust–mantle coupling beneath Tibet. In contrast to Tibet, the misfit between maximum shear and splitting directions in Yunnan is large. Neither the right-lateral (D/ s = 538) nor the left-lateral (D/ s = 498) maximum shear directions for the surface deformation field show a close correspondence to the shear wave splitting measurements in Yunnan (Fig. 4; Table 2), indicating that the crust and mantle layers are decoupled. This observation suggests that there is a fundamental difference between Tibet and Yunnan province. 5. Dynamic modeling of the mantle lithosphere beneath Asia We hypothesize that the difference between these two regions is that in contrast to the crust–mantle coupling in Tibet, there is complete decoupling in Yunnan. Thus the deformation field and associated anisotropy in the Tibetan mantle is a result of mantle boundary forces, as well as buoyancy forces that are transmitted through the crust, while the deformation field and associated anisotropy in the Yunnan mantle are a result of mantle boundary forces alone. This hypothesis is testable because it makes specific predictions for both Tibet and Yunnan. Since the decoupling removes the influence of crustal GPE variations on the mantle, then a mantle model including only boundary conditions should correctly account for the style of mantle deformation in Yunnan. In contrast, such a model should not work for Tibet, due to the influence of GPE variations there. We thus constructed a model for the entire collision zone in which the mantle is completely decoupled from the overlying crust (this is justified as long as there are negligible density variations within the anisotropic mantle lithosphere), so that it only includes the influence of boundary conditions through the prescribed motions of rigid geologic blocks, in the absence of body force variations. We first identify 5 L.M. Flesch et al. / Earth and Planetary Science Letters 238 (2005) 248–268 geologic blocks (Tarim Basin, Gobi platform, Odros block, south China block, and Sunda Block) that show relatively low strain rates in the GPS and lack active large-scale geological deformational structures (see outlined areas in Fig. 1). We then make the assumption that these 5 geologic blocks and the Indian plate are rigid throughout the vertical extent of the lithosphere. These rigid blocks surround the Tibetan Plateau and Yunnan (Fig. 1) and their relative motions are known from GPS (Fig. 2). We assume that the leading edge of the Indian plate is located in southern Tibet, where it has been imaged seismically [78]. The shapes and relative motions of these lithospheric blocks, as well as the leading edge of the Indian plate, provide precise velocity boundary conditions that indeed constrain the mantle deformation field beneath Tibet, which is surrounded by these blocks. Moreover, the resulting lithospheric deformation field is independent of any reference frame that can be adopted for the block motions and is only dependent on their relative motions. With the velocity boundary conditions applied to the 5 geologic blocks and the Indian plate we perform standard continuum-mechanics forward modeling to predict a mantle deformation field and resulting maximum shear directions within the upper mantle beneath Asia. Flesch et al. [12] have shown that the minimization of the rate of work functional Z Z H¼ ½ D ma fa dxdy ð2Þ s with respect to the strain rate and velocity fields provides a deviatoric stress field that solves the force balance equations [79,80]. In Eq. (2) f a is the body force distribution, m a is a velocity, and D is the dissipation potential given by: nþ1 n 2n B ėe ab ėe ab þ ėe 2cc D¼ ð3Þ nþ1 where n is the power law exponent, ė gg = (ė xx + e yy) = ė zz is the strain rate, and B is a constant related 1 to˙ the viscosity, ḡ ¼ BE n 1q, ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi that is sensitive to temperature [2,22] where E ¼ ėe ab ėe ab þ ėe 2cc . We follow the method of Flesch et al. [12] and use Eq. (2) to forward model the central Asian mantle for a decoupled lithosphere (free-slip top boundary) in the absence of body forces, assuming a uniform B value distribution and n = 1. We then apply the velocity boundary conditions of the 5 geologic blocks and the Indian plate, known 263 from the GPS measurements (see velocity vectors, Fig. 2) (Fig. 1) and calculate the resultant velocity, strain rate, and deviatoric stress field within the interior of the grid. As expected, the mantle strain rate field from this dynamic model is considerably different from the surface deformation field (Fig. 5; Table 2), due to the absence of crustal GPE variations. In Yunnan the left-lateral maximum shear directions for this model provide a very good fit to the splitting directions (D/ m = 88). This correlation provides further support for the hypothesis that deformation and observed anisotropy in the Yunnan mantle is driven exclusively by boundary conditions imposed from the edges of adjacent lithospheric blocks and is decoupled from the overlying crustal velocity field. For Tibet, the fit provided by the left-lateral maximum shear directions for this mantle model is degraded, compared to the fit to the surface deformation field, with the RMS misfit nearly doubling (D/ m = 218 vs. D/ m = 118) (Fig. 5). This is expected because this model lacks the topographically induced body forces (i.e., crustal GPE variations) that are required to explain the surface deformation field, as noted above [12]. There are three diagnostic stations in Tibet: the two eastern stations and the most southern station in the middle line (Fig. 5). These stations delineate a rotation or curvature of the fast polarization directions that is not present in the boundarycondition-only mantle deformation model. This lack of rotation lends further support to the conclusion that in the Tibetan mantle is deforming in response to the near-surface topography-induced forces (GPE variations) that are transmitted through the entire depth of the lithosphere. For this to occur there must be a mechanical coupling between the crust and upper mantle (Fig. 4, Table 2). In addition, the mantle layer cannot be stronger than the crustal layer. If the mantle is the strong controlling layer then any crustal buoyancy forces transmitted into the mantle would not influence mantle deformation and one would not expect the observed rotation of /. In order to model both the strong coupling in Tibet and weak coupling in Yunnan, we now consider a bhybridQ mantle model where we again impose rotations of lithospheric geologic blocks as boundary conditions, but now add a second constraint to approximate the top coupling boundary condition 264 L.M. Flesch et al. / Earth and Planetary Science Letters 238 (2005) 248–268 that the crustal velocity field in Tibet (Fig. 2) might impose on the mantle below. To approximate the effect of top coupling boundary conditions we again minimize the functional [Eq. (2)] with applied side boundary conditions, but with the additional constraint of fitting the surface strain in the vicinity of the splitting measurements within Tibet, but not within SE Tibet or Yunnan. This moderate constraint is different from the applied side velocity boundary conditions in that the model does not have to fit the top boundary conditions precisely in minimizing the functional in Eq. (2). We again use a uniform B value distribution and power law exponent of 1 and impose the rotation of each of the 5 geologic blocks and the Indian plate to solve for the resulting velocity, strain rate, and stress field within the deforming region (Fig. 6). This hybrid model produces a good fit in both Tibet (D/ m = 108) and Yunnan (D/ m = 98) regions to the values of / for the left-lateral case, with an overall misfit of D/ m = 98 (Table 2). This is consequently our preferred model. The velocity field of the dynamic hybrid solution for the mantle with side boundary conditions, and approximated top boundary conditions within Tibet only, is shown in Fig. 7 along with the surface velocity field. As expected, this model has minimal relative crust–mantle velocities beneath Tibet, but large differential horizontal velocities between crust and mantle beneath Yunnan. These differential horizontal velocities are as large as ~30 mm/yr in Yunnan where the crust is moving to the S–SE with respect to Eurasia, but the underlying mantle is being pushed NE–E by the effect of the collision. The transition between the primarily coupled crust–mantle motions in Tibet to the decoupled crust–mantle motions in Yunnan occurs in the vicinity of eastern Himalayan syntaxis. The resulting horizontal shear within Yunnan between the crustal and mantle layers predicts large N–S shear strain rates (ė yz) (Fig. 8), with the crust moving southward with respect to the mantle. 6. Discussion and conclusions We have modeled both kinematically and dynamically central Asian tectonics through the joint analysis of surface deformation (Fig. 2) inferred from observations of GPS and Quaternary fault slip data, and mantle deformation observations inferred from available SKS shear wave splitting observations, including a new data set for Yunnan (Fig. 1). We first used these data to determine whether the mantle anisotropy was due to lithospheric deformation or to flow in the asthenosphere by comparing observed splitting fast polarization directions to those predicted by simple asthenospheric flow. Asthenospheric flow was tested by predicting values of / under a variety of assumptions about the sub-asthenospheric velocity field. A sub-asthenospheric velocity field that is stationary in either the hotspot (Fig. 3A) or NNR (Fig. 3B) reference frame provides an unsatisfactory fit to the data, as does a best-fit model predicted by a uniform velocity rotation over the entire region (Fig. 3C). While allowing separate velocities under Tibet and Yunnan do provide a reasonable fit of data, the resulting flow field appears to be incompatible with several firstorder geophysical and geological constraints and is therefore unlikely to provide a physically reasonable explanation for the splitting data. We then investigated if the source of the anisotropy could be lithospheric. We initially compared the maximum shear directions derived from the observed surface deformation field with the splitting fast polarization directions (Fig. 4), which should be parallel to each other if the lithosphere is deforming coherently. For Tibet, we found a correspondence for the left-lateral maximum shear directions, but not for right-lateral. Both this close correspondence and the predominance of left-lateral surface faulting in Tibet, argue strongly for a lithospheric source for the mantle anisotropy. Previous modeling [12] has shown that the surface deformation field in Tibet can be successfully fit by a model that includes equal contributions of stress associated with the accommodation of relative plate motions and those associated with GPE variations within the lithosphere. As noted earlier, GPE variations are effectively concentrated within the crust if topography is compensated by the Airy mechanism [16]. Therefore, in order for crustal buoyancy forces in Tibet to influence mantle deformation, they must be transmitted through the crust and into the mantle, arguing for strong mechanical coupling between the crustal and mantle layers. In addition, the mantle layer cannot be stronger than the crustal layer. If this were the case then the transmitted crustal buoyancy forces L.M. Flesch et al. / Earth and Planetary Science Letters 238 (2005) 248–268 would have no influence on the mantle deformation field and / would not exhibit the observed curvature associated with crustal GPE variations [12]. In contrast, for Yunnan, neither left- nor right-lateral maximum shear directions from surface deformation successfully predict the values of /. We therefore reject the model of vertically coherent lithospheric deformation for Yunnan, and then considered the hypothesis that the crust and mantle portions of the lithosphere are mechanically decoupled. In this case the mantle deformation beneath Yunnan crust is dominated by the velocity boundary conditions of the lithospheric blocks that surround Yunnan, while the crustal topographically induced stresses are not transmitted to the mantle. In order to test this hypothesis, we constructed a dynamic mantle model that was subject only to velocity boundary conditions by imposing the relative motions of 5 undeformed geologic blocks as well as the motion the of Indian plate, with no body force variations within the mantle portion of the lithosphere. The resulting mantle model provides a good fit to the values of / in Yunnan for the left-lateral maximum shear direction. Given this correspondence, we view this as a successful model. Furthermore, we conclude that the Yunnan lithosphere is decoupled, and observed anisotropy in the Yunnan mantle is the result of strain associated with the motion of the surrounding lithospheric blocks. As expected, this decoupled, boundary-condition-only mantle model degrades the fit for Tibet, which provides further confirmation that the crust and mantle cannot be decoupled there; the observed coherence between crustal shear directions and / can only occur if crustal buoyancy forces are transmitted from the crust into the mantle beneath Tibet via mechanical coupling. We then constructed a bhybridQ mantle dynamic model for mantle deformation that accounts for side velocity boundary conditions of blocks as well as approximated top velocity boundary conditions associated with the motions of the overlying crust within regions where coupling between crust and mantle is hypothesized to be occurring. The top velocity boundary conditions were placed only within Tibet, but not within SE Tibet or Yunnan. This model provides an excellent fit to the overall splitting data set, and as a result, is our preferred model to explain deformation and vertical strength variations within the central Asian lithosphere. By comparing the mantle model 265 with the surface deformation field, assumed to be a proxy for the motion of the upper crust, we are then able to quantify the horizontal differential velocity between crust and mantle. As expected this difference is largest in Yunnan (reaching a maximum of ~30 mm/ yr), where the crust appears to be moving southward with respect to the mantle. In the hybrid model there is only imposed crust– mantle coupling out to the eastern edge of Tibet above 338 N, where there are splitting measurements to constrain the coupling. Thus a broad transition zone is permitted from 338 N in Tibet (where we observed the lithosphere to be coupled) to 268 N in a decoupled Yunnan. Preliminary shear wave splitting observations from a recent portable experiment in the eastern Himalayan syntaxis region [81] provide both additional support for the existence of this predicted transition and also a tighter bound on its location and spatial extent. We find that the coupled solution, (Fig. 4), successfully explains these additional data, suggesting that the Tibetan lithosphere is still coupled as far south as 298 N, which implies a narrow north– south transition zone of no more than ~300 km. In addition, recent shear wave splitting measurements south and east of the eastern Himalayan syntaxis argue for a decoupled lithosphere [82] further narrowing the width of this transition zone. The results for Tibet from the models described above provide three important conclusions concerning the vertical rheological profile of the lithosphere of Tibet. First, it implies a strong coupling between the crust and mantle within the Tibetan plateau. Secondly we infer that this coupling causes stresses associated with buoyancy forces originating in the upper crust, to be transmitted into the mantle. An upper crust that is substantially weaker than the mantle is not compatible with our results. Third, the coupling between crust and upper mantle is only possible if the lower crust and upper mantle are of similar strength within Tibet; thus a bjelly-sandwichQ rheology with a weak middle or lower crust can be ruled out for Tibet. In addition, our results are inconsistent with other forms of crust– mantle decoupling, such as large-scale lower crustal flow in Tibet [10,11,13] or large-scale delamination of the Eurasian mantle in northern Tibet [15,83]. The contrast between the behavior of Tibet and Yunnan is striking. The inferred change from strongly coupled to decoupled lithosphere suggests that there is 266 L.M. Flesch et al. / Earth and Planetary Science Letters 238 (2005) 248–268 a profound lateral transition in the strength profile of the lithosphere over the central Asian region. It implies that there are large horizontal shear strains (ė xz, ė yz, Fig. 8) in the transition region around the eastern Himalayan syntaxis [84], as if the Yunnan crust is sliding off the plateau, while the underlying mantle continues to be compressed by the collision. This change in vertical rheology between Tibet and Yunnan may be due to the different ways each region was formed, or to the specific way in which this particular orogen has evolved. As more shear wave splitting observations are made in this region, we will gain a much clearer view of the lithosphere’s actual role in the most dramatic of collisions. Acknowledgements We thank S. Cai, P. Burkett and W. Jiao for leading the installation and data collection of the Yunnan portable seismic network. 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